1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control system in an internal combustion engine having a single air-fuel ratio sensor downstream of or within a three-way reducing and oxidizing catalyst converter within an exhaust gas passage.
2. Description of the Related Art
In known air-fuel ratio feedback control systems using air-fuel ratio sensors (O.sub.2 sensors), there exist a single O.sub.2 sensor system having a single O.sub.2 sensor and a double O.sub.2 sensor system having two O.sub.2 sensors one upstream and one downstream of the catalyst converter. Note, in a single O.sub.2 sensor system, the O.sub.2 sensor is disposed either upstream or downstream of the catalyst converter.
In a single O.sub.2 sensor system having an O.sub.2 sensor upstream of the catalyst converter, the O.sub.2 sensor is disposed in an exhaust gas passage near the combustion chamber, i.e., near the concentration portion of an exhaust manifold, upstream of the catalyst converter. In this system, however, the output characteristics of the O.sub.2 sensor are directly affected by the non-uniformity or non-equilibrium of the exhaust gas. For example, when the air-fuel ratio actually indicates a rich state, but oxygen is still present, the output characteristics of the O.sub.2 sensor fluctuate. Also, in an internal combustion engine having a plurality of cylinders, the output characteristics of the O.sub.2 sensor are also directly affected by individual differences between the cylinders, and accordingly, it is impossible to detect the mean air-fuel ratio within the entire engine, and thus the accuracy of the controlled air-fuel ratio is low.
On the other hand, in a single O.sub.2 sensor system having an O.sub.2 sensor downstream of the catalyst converter, the non-uniformity or non-equilibrium of the detected exhaust gas can be eliminated, and the mean air-fuel ratio within the entire engine can be detected. In this system, however, due to the long distance between the O.sub.2 sensor and the exhaust valves, and because the capacity and cleaning efficiency of the catalyst converter depends upon its O.sub.2 storage effect, the response characteristics of the O.sub.2 sensor are lowered, thus reducing the response characteristics of an air-fuel ratio feedback control system. As a result, the efficiency of the catalyst converter cannot be sufficiently exhibited, thus increasing HC, CO, and NO.sub.x emissions.
Also, in the above-mentioned double O.sub.2 sensor system, an air-fuel ratio feedback control operation is carried out by the downstream O.sub.2 sensor in addition to an air-fuel ratio feedback control operation by the upstream O.sub.2 sensor. For example, the mean air-fuel ratio is detected by the downstream O.sub.2 sensor to reflect an air-fuel ratio feedback parameter such as skip amounts, and the air-fuel ratio feedback control for the entire system is carried out by the output of the upstream O.sub.2 sensor and the air-fuel ratio feedback control parameter (see U.S. Pat. No. 4,693,076). Therefore, even if the output characteristics of the downstream O.sub.2 sensor are not stable, good emission characteristics are obtained. In this double O.sub.2 sensor system, however, two O.sub.2 sensors are required, thus increasing the manufacturing cost, and further, when the frequency of the air-fuel ratio feedback control by the upstream O.sub.2 sensor is increased by the aging of the parts of the engine or the like, the efficiency of the catalyst converter is lowered.
In view of the foregoing, the present inventor has already suggested a single O.sub.2 sensor system having a downstream O.sub.2 sensor in which a self-oscillating term AF.sub.s having a definite amplitude and a definite period is generated, and a mean value (coarse-adjusting term) AF.sub.c of the self-oscillating term AF.sub.s is changed in accordance with the output of the downstream O.sub.2 sensor, to thereby exhibit full efficiency of the catalyst converter (see Japanese Unexamined Patent Publication (Kokai) No. 64-66441 published on Mar. 31, 1989).
On the other hand, in a vehicle where the catalyst converter frequently cannot exhibit the required O.sub.2 storage effect if the air-fuel ratio upstream of the catalyst converter is greatly deviated from the stoichiometric air-fuel ratio for a long time, the O.sub.2 storage effect of the catalyst converter is different from that in a cruising state, and thus the O.sub.2 storage effect cannot be ensured, thereby reducing the accuracy of the control of the air-fuel ratio. Accordingly, the present inventor also suggested the introduction of an O.sub.2 storage term corresponding to the O.sub.2 storage amount of the catalyst converter into the control of the air-fuel ratio (see U.S. Ser. No. 487454).
Nevertheless, even when the O.sub.2 storage term is introduced into the control of the air-fuel ratio, it is impossible to compensate for the reduction of the O.sub.2 storage effect of the catalyst converter in a warming-up mode, thus increasing HC and CO emissions in the warming-up mode.
That is, in the warming-up mode, a warming-up incremental fuel is supplied to the engine, to enrich the controlled air-fuel ratio, thus compensating for the friction of the engine. Also, in the warming-up mode, the amount of fuel adhered to the walls of an intake air passage and the like is so large that the controlled air-fuel ratio is remarkably rich even in an after-warming-up mode, i.e., an acceleration mode or a deceleration mode. Thus, in the warming-up mode and in the after-warming-up mode, the O.sub.2 storage amount of the catalyst converter is remarkably reduced. When this remarkable reduction of the O.sub.2 storage amount is compensated for by the introduction of the O.sub.2 storage term AF.sub.CCRO into the control of the air-fuel ratio, the O.sub.2 storage term AF.sub.CCRO on the lean side is cleared by a temporary lean output of the O.sub.2 sensor in the warming-up mode, even if the controlled air-fuel ratio is originally rich, which will be later explained, and as a result, it is impossible for the controlled air-fuel ratio to reach a desired lean air-fuel ratio, to increase HC and CO emissions. Contrary to this, if the O.sub.2 storage term AF.sub.CCRO is not cleared by the inversion of the output of the O.sub.2 sensor, the convergence characteristics of the controlled air-fuel ratio in the after-warming-up mode are degraded.